1. Field of the Invention
The present invention relates to an apparatus for monitoring optical signal performance for operation/maintenance/management of optical communication networks. More particularly, the invention relates to a monitoring apparatus detecting optical signal distortions due to polarization-mode dispersion and chromatic dispersion and indicating the magnitudes of polarization-mode dispersion and chromatic dispersion by means of measuring the power over the signal frequency band of interest and using the polarization scrambling technique at the sending-end.
2. Description of the Related Art
Wavelength division multiplexing (WDM) scheme based optical networks are efficient high-speed broadband networks that transmit high-speed optical signals by allocating a plurality of wavelengths on one communication channel. In the recent information society, as demand for various kinds of data services such as Internet and high-quality video service has shown a dramatic increase, data transfer for these services has begun to require high-speed and large capacity broadband optical networks. To accommodate this type of data, transmission rate per channel in WDM optical networks has increased accordingly. In high-speed optical networks, optical signal distortion due to the polarization-mode dispersion and the chromatic dispersion occurring in an optical fiber affects the quality of optical signals. The polarization-mode dispersion limits the allowable maximum transmission distance and aggravates transmission quality by causing expansion of pulse width of the light signal being modulated into the square wave along with the chromatic dispersion in optical transmission systems. Polarization-mode dispersion and chromatic dispersion are ever becoming a serious problem as the networks demand larger capacity and higher speed.
Polarization-mode dispersion and chromatic dispersion might change by a large amount in case of network reconstruction such as replacing optical fiber and also they may change from time to time as the temperature of the network environment changes. For example, the total dispersion of a 500 km long LEAF (Large Effective Area Fiber) can change as much as 80 ps/nm for a temperature change of 40 degrees. This amount exceeds the permissible dispersion of a system having channel transmission rate of 40 Gb/s. Also, the polarization-mode dispersion of optical fiber undergoes stochastic changes due to fiber's structural incompleteness, pinch, bending, twist, pressure, temperature, etc. And it is probabilistically known to be about 21 minutes a year when the temporal value of polarization-mode dispersion is larger than 3 times the year-average value. This much probability can cause a little bit of penalty to a system with channel transmission rate of 2.5 Gb/s when existing fibers with a large polarization-mode dispersion is used for a long transmission distance of 640 km. It can also cause a little bit of penalty to a system using currently available low polarization-mode dispersion fiber if the channel transmission rate reaches 40 Gb/s. Therefore, it is apparently needed to constantly monitor the polarization-mode dispersion and the chromatic dispersion of optical signal in high-speed optical networks.
The techniques for measuring the polarization-mode dispersion of optical elements in the prior art include; a method in which polarization change is measured against wavelength of a light source after passing the light source with a fixed polarization through an object element of interest for measuring, and a method in which change of the polarization state of the light is directly observable as to the light wavelength using wavelength tunable laser and polarimetric analyzer.
Recently, a method has been proposed to monitor distortion of optical signals due to polarization-mode dispersion when the light signal passes through optical fibers having polarization-mode dispersion in optical networks. In this method the signal distortion due to polarization-mode dispersion is monitored by measuring the change of power over the signal frequency band that is caused by the distortion of optical signals due to polarization-mode dispersion.
The aforementioned prior art is about methods to simply measure the polarization-mode dispersion of optical fibers or to monitor the signal distortion due to polarization-mode dispersion. But these methods are unable to monitor the polarization-mode dispersion value of the optical signal when it passes through various optical fibers. So far, no methods of monitoring polarization-mode dispersion for these cases have been disclosed. Accordingly, the methods in the prior art could not discern other factors such as chromatic dispersion which affects distortion of optical signals. As the optical signal passes through optical fibers, the phase of each frequency component is modulated due to the chromatic dispersion of optical fibers. If this signal is photoelectrically converted in the light receiver, the power of electric signals changes according to the chromatic dispersion value of optical fibers. Consequently, the monitoring methods in the prior art were not able to discern the optical signal distortion caused by polarization-mode dispersion from that by chromatic dispersion.